It has become increasingly common to manufacture mechanical components, such as for instance, but not limited to, turboengine components, from material powders by means of additive manufacturing methods which are similar to rapid prototyping. In applying such methods, no specific tooling for a component is required. Generally, said methods are based upon depositing a material powder, for instance a metal powder, and melting and resolidifying the powder at selected locations such as to form a component with a specific geometry from the resolidified material. As is apparent, these methods allow for a great flexibility of the geometry of the component to be manufactured, and allow for instance undercuts, manufacturing almost closed cavities, and the like. In particular, the powder is deposited layer by layer, each layer measuring for instance in the range of some tenths of a millimeter. The melting step is performed such as to locally melt the powder and the surface of a solidified solid volume beneath, such that the newly molten material is, after resolidification, substance bonded to an already manufactured solid volume. Such methods are for instance known as Selective Laser Melting (SLM) or Electron Beam Melting (EBM), while not being limited to these methods.
Mechanical components manufactured by a method of the above-mentioned kind exhibit largely different microstructures compared to conventionally cast or wrought components made from the same alloy. For instance, when compared to cast or wrought components, the grain structure is significantly finer, due to the high cooling rate which is present during, for instance, SLM or EBM processing. Moreover, when applying additive manufacturing from nickel or gamma prime forming cobalt based superalloys, the high cooling rates also cause a gamma prime or γ′ free microstructure after the additive manufacturing process. With respect to cobalt based alloys, cobalt based superalloys in which, according to current knowledge, tungsten and/or aluminum are contained as alloying elements have growingly become of practical interest and have shown to form gamma/gamma prime microstructure. When an accordingly manufactured article from such superalloys is heated up the first time to sufficiently high temperatures, the gamma prime phase starts to precipitate. It is in particular the gamma prime phase which causes the superior material properties with respect to elevated temperature strength and resistance to creep deformation. The gamma prime phase however exhibits a comparatively lower ductility when compared to the gamma phase. For instance, different thermal expansion within a component may result in high local stress intensities.
In order to achieve the named superior material properties at elevated temperatures, the component manufactured by additive methods commonly undergoes a heat treatment step in order to precipitate the gamma prime phase. During gamma prime precipitation the lattice parameter changes. That is, the volume occupied by a lattice changes when fractions of the gamma phase, γ, transform to the gamma prime phase, γ′. This dimensional change causes stresses in the component. For instance, it is observed that the lattice parameter of the gamma prime phase is in most instances smaller than the lattice parameter of the gamma phase. Thus, volume fractions of a component containing gamma prime precipitates shrink during a heating up process, whereas other volume sections expand. The lattice mismatch between gamma prime and surrounding gamma may result in significant stresses, which are further amplified by the low ductility of the gamma prime phase and are adding to residual stresses from manufacturing and stresses due to temperature gradients during heating up of the component. Said combined stresses may result in crack initiation when heating up the component for the first time, also referred to strain age cracking, or SAC, and related elevated scrap rates.
The teaching of EP 2 754 515 proposes to overcome this issue by adding crack resistant features to the manufacturing component. However, particularly in sharp transition areas or in areas which notch effects, these design changes may not be sufficient to avoid strain age cracking. Moreover, adding to some extent crack resistant features reduces the degree of freedom of design and the benefit of applying the specific manufacturing method.
EP 2 586 548 proposes to use a material wherein the grain size is controlled. EP2 589 449 strives at controlling heat intake into the additively manufactured component during the build process in order to reduce stresses. Other disclosures, such as in EP 2 737 965, EP 2 865 465, and EP 2 886 225, propose tailoring the material properties of the material used to the particular load conditions.
EP 2 815 841 focusses on the post-build heat treatment process.